Iran's Supreme Leader Ayatollah Ali Khamenei has launched a Twitter rant against Saudi Arabia for its air strikes against Shi'ite Houthi rebel group in Yemen.

Denouncing the Saudi-led air bombings of Houthi positions in Yemen, Khamenei called on Riyadh to stop "these criminal acts:

Iran has denied allegations that it is providing military support to the Houthis. The Islamic Republic sent two navy vessels to the Gulf of Aden, off the southern coast of Yemen to "safeguard naval routes for vessels in the region".

US secretary of State John Kerry warned Iran over its alleged support for the Houthi rebels, who are being driven back by a Saudi-led coalition which seeks to restore President Abdrabbu Mansour Hadi, who fled the country last month.

"There are obviously supplies that have been coming from Iran. There are a number of flights every single week that have been flying in," Kerry said on PBS Newshour.

"Iran needs to recognise that the US is not going to stand by while the region is destabilised or while people engage in overt warfare across lines, international boundaries and other countries," he added.

Fighting increased in the city of Aden between the Houthis and militias loyal to Hadi. Iranian president Hassan Rouhani also appealed to countries in the region to stop the air raids and solve the crisis.

"A great nation like Yemen will not submit to bombing. Come, let us all think about ending war. Let us think about a ceasefire," he said in a speech. "Let us prepare to bring Yemenis to the negotiating table to make decisions about their future."

Khamenei's Twitter statement follows earlier comments on his website about the framework nuclear deal struck in Lausanne. The 75-year-old said there is "no guarantee" on a final agreement with world powers and said he is "neither for or against it". "Everything is in the details, it may be that the deceptive other side wants to restrict us in the details," Khamenei said in a live address on state television.